This invention relates to a novel process for preparing piperonal.
Piperonal is the basis of heliotrope type perfume compositions and is widely used in perfumes for general cosmetic preparations. In addition, it is very useful as an industrial material for the production of drugs, agricultural chemicals, and the like and as a brightener for metal plating. Industrially, piperonal is now being produced mainly by isolating safrole from the essential oil of Ocotea cymbarum and oxidizing it with ozone or dichromate. In recent years, however, the tendency toward the exhaustion of natural resources from which safrole can be obtained has created a growing demand for stable supply of piperonal through its industrial synthesis from petroleum chemicals.
Conventional processes for preparing piperonal can be divided into two groups: one starting with 1,2-methylenedioxybenzene prepared from pyrocatechol and the other starting with 3,4-dihydroxybenzaldehyde. The former is found to be more advantageous to industrial applications.
Several processes for preparing piperonal from 3,4-dihydroxybenzaldehyde are known.
(1) In order to prepare the starting material or 3,4-dihydroxybenzaldehyde, a process has been proposed which involves condensing pyrocatechol with glyoxylic acid in aqueous alkali and then oxidizing the resulting dihydroxymandelic acid (Japanese Patent Application Disclosure No. 2,952/'75). The yield attainable with this process is at most 77%.
(2) One process for preparing piperonal from 3,4-dihydroxybenzaldehyde comprises reacting 3,4-dihydroxybenzaldehyde with methylene chloride and alkali in a non-protic polar solvent such as dimethyl sulfoxide, and gives a 61% yield of piperonal (British Pat. No. 1,097,270). Another process comprises reacting 3,4-dihydroxybenzaldehyde with methylene chloride in aqueous alkali under the influence of an interphasic moving catalyst such as quaternary ammonium compound, and gives a 70-73% yield of piperonal. However, the catalyst used in this process is expensive and, by nature, easily soluble in both aqueous and organic phases. Accordingly, the recovery of the catalyst is too low to make this process practicable (Japanese Patent Application Disclosure Nos. 23,265/'76 and 113,967/'77). In any event, neither of these process can provide satisfactory yields based on the amount of the starting material. In addition, James H. Clark et al. have described still another process in which 3,4-dihydroxybenzaldehyde is reacted with dibromomethane in N,N-dimethylformamide under the influence of an excess of potassium fluoride or cesium fluoride to give a 90% yield of piperonal [Tetrahedron Letters, No. 38, pp. 3361-3364 (1976)]. This process is of advantage in that no strong base is used, the reaction time is relatively short, and a high yield can be obtained. However, an expensive fluoride must be used in large excess as a halogen trapping agent and cannot be recycled because its fluorine component is stoichiometrically replaced by the halogen atoms in the dihalomethane used. Accordingly, this process is not satisfactory for industrial applications. Moreover, when based on the amount of pyrocatechol used, the yield attainable with this process does not reach 70%.
Meanwhile, several processes for preparing piperonal from 1,2-methylenedioxybenzene are also known.
(1) The starting material or 1,2-methylenedioxybenzene can be prepared by processes in which pyrocatechol is reacted with methylene chloride and alkali in a non-protic polar solvent such as dimethyl sulfone to give a 91-99% yield (British Pat. No. 1,097,270 and Japanese Patent Application Disclosure Nos. 5,963/'76 and 13,773/'76). In addition, other processes which can give high yields of 1,2-methylenedioxybenzene are also available. Accordingly, it would be of great advantage from an industrial viewpoint if piperonal could be prepared from 1,2-methylenedioxybenzene with good yield and high selectivity.
(2) A typical process for preparing piperonal from 1,2-methylenedioxybenzene has been reported by P. P. Shorygin et al. [J. Gen. Chem. (U.S.S.R.), 8,975 (1938)]. This is a two-step process. In the first step, 1,2-methylenedioxybenzene is reacted with formalin in petroleum benzine under the influence of hydrogen chloride gas and zinc chloride to form piperonyl chloride (with a 70-78% yield based on the amount of 1,2-methylenedioxybenzene having reacted). This is followed by the second step in which the piperonyl chloride is reacted with an equimolar amount of hexamine in 60% alcohol to give a 70-80% yield of piperonal. By E. D. Laskina et al. [Chemical Abstracts, 57,9714 (1962)], another process has been described in which 1,2-methelenedioxybenzene is reacted with formalin in the presence of a large excess of the sodium salt of m-nitrobenzenesulfonic acid, hydrogen chloride, and an aluminum catalyst to give a 42.4% yield of piperonal.
However, these conventional processes are not entirely satisfactory for industrial applications because the yield and particularly selectivity (that is, the yield of piperonal based on the amount of 1,2-methylenedioxybenzene having reacted) is rather limited due to the abundant formation of tarry by-products, the procedure for reaction is complicated, and the large amount of metal hydroxide formed as a by-product must be disposed of.